1. Field of the Invention
This invention is directed to an optical device for scanning and interpreting (reading) symbols on a distant surface. Typical symbols which can be scanned and interpreted are bar codes, numbers, letters, characters and the like.
2. Description of the Related Art
There are now various types of optical scanning devices. Two of the most well known types of optical scanning devices are bar code readers and optical character readers (OCR). In general, optical scanning devices scan a surface and interpret symbols such as, for examples, bar codes, letters, numbers and characters. In the case of certain optical scanning devices, such as an OCR, the surface being read is always at a predetermined position. Typically, a document being scanned is placed on a glass that is at a fixed position. An optical system of the OCR knows exactly where the document is located. Its optical system can be optimized for reading the paper at a fixed distance. However, there are situations in which an optical scanning device must read symbols which are not located at a predetermined distance from the scanning device, and may be moving with respect to the scanning device. For example, it may be desirable to read symbols affixed to objects being transported on a moving conveyor belt. The symbols must be read without any part of the scanning device coming into physical contact with the symbol's surface. Additionally, the distance between the optical scanner and the symbol being scanned can vary, depending upon the physical dimensions of the object to which the symbol is attached.
In one known arrangement for scanning a remote surface, a laser beam is directed toward a polygon mirror or a galvano mirror which rotates about an axis. As the mirror rotates the symbol surface is scanned by the laser light reflected from the rotating mirror. The laser light may be light from a semiconductor laser light source transmitted through a condensing lens. Laser light reflected from both the symbol and the surface on which the symbol is affixed is detected utilizing photo detecting means as optoelectric transducing elements. In the case of a scanner for scanning a bar code on a remote surface, a bar code normally includes a series of spaced apart white and black lines, called "bars", of various lengths and widths. Because the laser beam is directed at a rotating, multisurfaced mirror, the reflected light moves across the series of bars, thereby "scanning" the symbols which comprise the code. The laser beam travels in a direction orthogonal to the individual bars which make up a bar code. The photo detecting means receives intensive light reflected from white bars, and less intensive light reflected from black bars and generates an electrical signal indicative of intensity. This electrical signal is processed into a two-level signal (a signal having one of only two possible levels) based on some predetermined function of intensity. In this manner, the optical scanner "reads" the bar code.
A "read distance" is the distance between the symbol being scanned and the scanning device within which the device can adequately read the symbol. The range between the lower limit and the upper limit of the scanner's read distance is called the scanner's "read range". It is desirable that this read range be as large as possible.
In the type of symbol reader that contains a condensing lens for condensing light beams emitted from a semiconductor laser light source, maximizing the read range presents a difficult design tradeoff. If the focal point of the condensing lens is set to infinity (infinite focal length), so as to obtain a perfectly paraxial light beam, it is not possible to reduce the diameter of the light beam enough to allow for adequate resolution of the symbols (even using a laser beam). On the other hand, if the condensing lens is set to a predetermined short focal length, so as to produce a very small beam at the surface to be read, when the surface to be read is even slightly displaced from the lens focal point, the beam will become so wide as to degrade the resolving power of the scanner.
In typical known bar code readers, the focal point is generally set at or near the center of the read range. The bar code reader designed in this manner, however, has various operational problems. When the optical scanner is operated too close to the symbol surface, the scan width of the laser beam on the symbol surface is small. Thus, the visual field of the bar code reader becomes narrow. The result is that under this condition, the bar code reader fails to read "long" bar codes. On the other hand, when the optical scanner is operated at too great a distance from the symbol to be read, the diameter of the laser beam on the symbol surface, is so large that the resolving power of the lens is poor, and the bar code reader cannot read "narrow" bar codes. In order for an operator to find an optimum read range for reading narrow bar codes, he must manually adjust the distance between the scanner and the symbol being scanned or manually adjust the lens. Such a requirement reduces worker efficiency, because the operator must spend additional time making such adjustments. When the bar code reader used is of the hand-held type, work efficiency is remarkably reduced. In the case of a bar code reader that is fixedly installed, the work required to adjust the read range is quite complicated.
One technique for attempting to solve this problem is disclosed in Japanese Patent Unexamined Publication No. 63-83886. An infrared light emitting diode (LED) emits rays of light toward the symbol surface of an object. A photo sensitive diode (PSD) receives the reflected light from the symbol surface. A distance between the bar code reader and the symbol surface is then measured from the detected positions of the reflected light (triangle measurement). In this known arrangement, a condensing lens is used to condense the reflected light from the symbol surface and to form a bar code image on the two dimensional image sensor. An optical position of the condensing lens is changed according to the measured distance data. Thus, the lens is automatically focused. In this manner, the read range is widened. The distance measuring means, including the LED and PSD, are essential elements to the scanning device. Such measuring means add to the overall cost of the scanning device. Moreover, power dissipation is increased, thus making the disclosed apparatus unsuitable for a Thus, it would be advantageous to utilize a scanning device which does not require the LED or the PSD.
Another known arrangement is set forth in U.S. Pat. No. 4,818,886 directed to a bar code reader. Optical elements, such as a light source, photo sensor, stop member and lens are controlled in order to obtain an exact read of bar codes. Resolving power is improved by fabricating a condensing lens from a resilient material. The curvature of the lens is varied by a tubular piezoelectric element. This element is used to change the focal distance of the condensing lens, thereby improving the lens resolving power (see FIG. 4 of U.S. Pat. No. 4,818,886).
There are several disadvantages to this technique, however. First, the piezoelectric element used must be specially shaped. The special shaping results in added cost. Additionally, the drive technique used to drive the element is complicated.
U.S. Pat. No. 4,818,886 discloses one arrangement for resolving the changing read distance problem. The position of the light source is changed by selectively energizing an infrared light emitting diode (LED) from among those of an LED array that are obliquely disposed with respect to an optical axis (see FIGS. 12 and 13 of the patent). However, an image forming position on the photo sensor is not coincident with the image forming the LED on the optical axis is selected. Thus, an exact read can not be obtained.